This application claims the priority of Japanese patent document 10-127668, filed May 11, 1998, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a molten metal plating apparatus in which an object is plated by immersing it in a molten metal, and particularly to a molten metal plating apparatus composed of components having good corrosion resistance and wear resistance to the molten metal.
Metals such as casting iron, stainless steel, high chromium steel and the like having corrosion resistance have been used for components of a molten metal plating apparatus, which are disposed in a molten metal. However, sinking rollers, supporting rollers and the like which are made of these materials cannot be used for long periods of time because the molten metals are strongly corrosive. Further, when a roller bearing or the like is corroded or worn, a plated film cannot be formed uniformly because of occurrence of vibration in a steel plate, which sometimes deteriorates the plating quality.
When an iron component used in a molten metal is corroded by the molten metal, an impurity in the form of a chemical compound of iron and the molten metal (called "dross") is formed in the molten metal. This not only degrades the plating film quality, but also shortens the lifetime of the molten metal itself. For these reasons, the rollers in a molten metal plating apparatus must be exchanged frequently, and the productivity of plated products is poor due to interruption of the operation for exchange of the roller.
In order to solve these problems, one known technique is to cover the components used in the molten metal with a cermet or a ceramic which is resistant to corrosion by the molten metal. Alternatively, the whole component used in the molten metal can be made of the cermet or the ceramic material. For example, Japanese Patent Application Laid-Open No. 61-37955 discloses a method of manufacturing a roller for use in a molten metal bath, having good corrosion resistance, heat resistance and wear resistance, by plasma spraying a ceramic onto a surface of a steel body. Japanese Patent Application Laid-Open No. 4-124254 discloses a bearing made entirely of a cermet or a ceramic, while Japanese Patent Application Laid-Open No. 5-44002 discloses a shaft of a roller made of cermet and a bearing totally made of a ceramic.
However, in known technology for surface treatment by the plasma spray method, pin holes are formed in the melt-sprayed film, and the molten metal can penetrate through the pin holes to corrode the body material, therefore, the melt-sprayed film is easily peeled off from the body material, and accordingly the reliability is low. On the other hand, since the component which is to be placed in the molten metal is generally quite large , it is technically and economically difficult to make the whole component using a cermet or a ceramic.